It’s important to keep in mind that sometimes, your user base is part of your product. It takes certain amount of users to actually reach MVP.

Questions like…

How many users does it take to get the ecosystem going?

How many avg. # of friends/followers do you need to get the product run on its own?

How many avg. comments per post do you need to get the engagement up

… and so forth

There’s no one magic number for these kinds of questions, but obviously there will be a key threshold for almost all products that need some sort of networking effect to kick-in based on their product’s DNA.